This invention relates to a process for driving out or removing free water from flue gas gypsum. Flue gas gypsum is calcium sulfate in substantially dihydrate form (CaSO.sub.4.2H.sub.2 O). Hereinafter, dry flue gas gypsum, means CaSO.sub.4.2H.sub.2 O without free water associated therewith.
One prior art process for driving out water from flue gas gypsum is discussed in DOS No. 2,727,544, according to which a flue gas stream discharged from a dust extractor is fed in its entirety to a fluidized-bed dryer as both the drying and carrier gas. In the fluidized-bed dryer, the flue gas stream forms a fluidized layer together with the flue gas gypsum introduced into the dryer. The gas and flue gas gypsum mixture is then discharged from the fluidized-bed dryer and is separated in a dust precipitator connected downstream thereof. As a result of this process, the flue gas gypsum is dewatered to the hemihydrate (2CaSO.sub.4.H.sub.2 O), and discharged from the dust precipitator as the product, while the dust-free flue gas, cooled to a temperature of about 100.degree. C., is introduced into a wet desulfuration device.
In accordance with this prior-art process, the flue gases are fed into the fluidized-dryer at a temperature of 150.degree.-200.degree. C., and are removed therefrom at a temperature of about 100.degree. C. As a consequence, the flue gas gypsum is heated in the fluidized bed to a temperature of 130.degree.-150.degree. C. However, as is already known, in this process, the anhydrite (CaSO.sub.4) is also formed at this temperature from the flue gas gypsum, i.e., the dihydrate (CaSO.sub.4.2H.sub.2 O), in addition to the flue gas gypsum hemihydrate. Thus, the presence or formation of anhydrite results in a nonuniform product being obtained. Furthermore, there is also the danger in the conventional process, due to the cooling of the entire flue gas stream to a temperature of about 100.degree. C., that the temperature will fall below the acid dew point temperature in the flue gas, at least on the way from the dryer to the wet desulfuration installation, resulting in corrosion damage occurring on all units with which the flue gas comes in contact. This effect can only be avoided by heating the flue gas stream withdrawn from the dryer to a temperature of at least 120.degree. C. by supplying heat from an external source.
Various other possibilities have been considered for drying out free water from flue gas gypsum by utilizing the heat contained in dedusted flue gases without requiring an external energy source.